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Visscher C.,Lunar and Planetary Institute | Moses J.I.,Space Science Institute
Astrophysical Journal | Year: 2011

We explore CO →↔ CH4 quench kinetics in the atmospheres of substellar objects using updated timescale arguments, as suggested by a thermochemical kinetics and diffusionmodel that transitions from the thermochemical-equilibrium regime in the deep atmosphere to a quench-chemical regime at higher altitudes. More specifically, we examine CO quench chemistry on the T dwarf Gliese 229B and CH4 quench chemistry on the hot-Jupiter HD 189733b. We describe a method for correctly calculating reverse rate coefficients for chemical reactions, discuss the predominant pathways for CO →↔ CH4 interconversion as indicated by the model, and demonstrate that a simple timescale approach can be used to accurately describe the behavior of quenched species when updated reaction kinetics and mixing-length-scale assumptions are used. Proper treatment of quench kinetics has important implications for estimates of molecular abundances and/or vertical mixing rates in the atmospheres of substellar objects. Our model results indicate significantly higher Kzz values than previously estimated near the CO quench level on Gliese 229B, whereas current-model-data comparisons using CH4 permit a wide range of Kzz values on HD 189733b. We also use updated reaction kinetics to revise previous estimates of the Jovian water abundance, based upon the observed abundance and chemical behavior of carbon monoxide. The CO chemical/observational constraint, along with Galileo entry probe data, suggests a water abundance of approximately 0.51-2.6× solar (for a solar value of H2O/H2 = 9.61 × 10-4) in Jupiter's troposphere, assuming vertical mixing from the deep atmosphere is the only source of tropospheric CO. © 2011. The American Astronomical Society. All rights reserved.

Kiefer W.S.,Lunar and Planetary Institute
Journal of Geophysical Research E: Planets | Year: 2013

The Marius Hills, the Moon's largest volcanic dome field, has more than 250 basaltic domes and cones in an area 200 × 250 km across. It is a major free-air gravity anomaly, 236 mGal in the north and 150 mGal in the south. In the northern half of the structure, the topography can only explain about half of the gravity anomaly, and in the south, there is virtually no topographic relief associated with the gravity anomaly. High-density material must be present at depth, most likely as mare basalt intruded into the underlying porous feldspathic highland crust. The gravity anomaly is modeled using two spherical caps. The northern cap is 160-180 km in diameter and at least 3.0 km thick. The southern cap is 100-140 km in diameter and at least 6.2 km thick. The intruded basalt may have served as the magma chambers that fed the overlying surface volcanism. Magma crystallization within these chambers provided a source of crystal-rich, high viscosity lava that fed the volcanic domes. The volume of intruded basalt is 1.6 × 104 km3. The total volcanic volume, including both intruded and extruded material, is 2.6 × 10 4 km3, indicating that the Marius Hills is a major volcanic center. Intrusion of hot magma may cause thermal annealing of the porous feldspathic host rock, significantly reducing the host rock porosity. This would allow a large volume of magma to be intruded into the crust with little change in overall crustal volume. © 2012. American Geophysical Union. All Rights Reserved.

Norman M.D.,Lunar and Planetary Institute | Norman M.D.,Australian National University | Nemchin A.A.,Curtin University Australia
Earth and Planetary Science Letters | Year: 2014

A sharp rise in the flux of asteroid-size bodies traversing the inner Solar System at 3.9 Ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments. The prevalence of ~3.9Ga crystallization ages for lunar impact-melt breccias and U-Pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short-lived, cataclysmic episode of late heavy bombardment at that time. Here we report U-Pb isotopic compositions of zirconolite and apatite in coarse-grained lunar melt rock 67955, measured by ion microprobe, that date a basin-scale impact melting event on the Moon at 4.22 ± 0.01Ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later. Significant impacts prior to 3.9 Ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish. Other isotopic evidence such as 40Ar-39Ar ages of granulitic lunar breccias, regolith fragments, and clasts extracted from fragmental breccias, and Re-Os isotopic compositions of lunar metal is also suggestive of impact-related thermal events in the lunar crust during the period 4.1-4.3 Ga. We conclude that numerous large impactors hit the Moon prior to the canonical 3.9 Ga cataclysm, that some of those pre-cataclysm impacts were similar in size to the younger lunar basins, and that the oldest preserved lunar basins are likely to be significantly older than 3.9 Ga. This provides sample-based support for dynamical models capable of producing older basins on the Moon and discrete populations of impactors. An extended period of basin formation implies a less intense cataclysm at 3.9 Ga, and therefore a better opportunity for preservation of early habitable niches and Hadean crust on the Earth. A diminished cataclysm at 3.9 Ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance. © 2013 Elsevier B.V.

Kiefer W.S.,Lunar and Planetary Institute
Planetary and Space Science | Year: 2012

Reliable measurements of the Moons global heat flow would serve as an important diagnostic test for models of lunar thermal evolution and would also help to constrain the Moons bulk abundance of radioactive elements and its differentiation history. The two existing measurements of lunar heat flow are unlikely to be representative of the global heat flow. For these reasons, obtaining additional heat flow measurements has been recognized as a high priority lunar science objective. In making such measurements, it is essential that the design and deployment of the heat flow probe and of the parent spacecraft do not inadvertently modify the near-surface thermal structure of the lunar regolith and thus perturb the measured heat flow. One type of spacecraft-related perturbation is the shadow cast by the spacecraft and by thermal blankets on some instruments. The thermal effects of these shadows propagate by conduction both downward and outward from the spacecraft into the lunar regolith. Shadows cast by the spacecraft superstructure move over the surface with time and only perturb the regolith temperature in the upper 0.8 m. Permanent shadows, such as from thermal blankets covering a seismometer or other instruments, can modify the temperature to greater depth. Finite element simulations using measured values of the thermal diffusivity of lunar regolith show that the limiting factor for temperature perturbations is the need to measure the annual thermal wave for 2 or more years to measure the thermal diffusivity. The error induced by permanent spacecraft thermal shadows can be kept below 8% of the annual wave amplitude at 1 m depth if the heat flow probe is deployed at least 2.5 m away from any permanent spacecraft shadow. Deploying the heat flow probe 2 m from permanent shadows permits measuring the annual thermal wave for only one year and should be considered the science floor for a heat flow experiment on the Moon. One way to meet this separation requirement would be to deploy the heat flow and seismology experiments on opposite sides of the spacecraft. This result should be incorporated in the design of future lunar geophysics spacecraft experiments. Differences in the thermal environments of the Moon and Mars result in less restrictive separation requirements for heat flow experiments on Mars. © 2011 Elsevier Ltd. All rights reserved.

Bogard D.D.,Lunar and Planetary Institute
Chemie der Erde - Geochemistry | Year: 2011

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5Ga ago, the K-Ar chronometer rarely gives times of meteorite formation. Instead, K-Ar ages obtained by the 39Ar-40Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar-Ar dating technique. It then reviews Ar-Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar-Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ~200Ma after parent body formation, consistent with parent bodies of ~100km diameter. For a suite of H-chondrites, Ar-Ar and U-Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites-lodranites, although metamorphosed to higher temperatures than chondrites, give Ar-Ar ages which cluster tightly at ~4.51Ga. Ar-Ar ages of silicate from IAB iron meteorites give a continual distribution across ~4.53-4.32Ga, whereas silicate from IIE iron meteorites give Ar-Ar ages of either ~4.5Ga or ~3.7Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar-Ar and I-Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar-Ar ages of ~3.9Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ~0.2°C/Ma. Ar-Ar ages of a large number of brecciated eucrites range over ~3.4-4.1Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar-Ar ages of either >3.5Ga or <1.0Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar-Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar-Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) 40Ar. This excess 40Ar derives from shock-implanted martian atmosphere or from radiogenic 40Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I-Xe and U-Pb) and the oldest measured Ar-Ar ages are consistent with previous suggestions that the 40K decay parameters in common use are incorrect and that the K-Ar age of a 4500Ma meteorite should be possibly increased, but by no more than ~20Ma. © 2011 Elsevier GmbH.

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